Veneer drying methods



United States Patent VENEER DRYING METHODS Edward H. Willis, Seattle,Wash., assignor of one-third to Oscar C. Sundsby, Seattle, Wash.

No Drawing. Application November 4, 1949, Serial No. 125,647

13 Claims. (CI. 34-95) This invention relates to veneer drying. Moreparticularly, it pertains to a method for rapid heat-drying of rotarycut veneer, commencing with the wood substantially in the green stateand continuing until dry, and the general object is to accomplish thisrapid drying operation without rendering the veneer brash, causingobjectionable cracking and warping of the veneer, well known majorsources of waste. The method is to be distinguished, both as to purposeand effect, from the oil bath treatment of wood which is alreadysubstantially dried.

The difficulty of drying green veneer without causing suchwaste-producing defects becomes especially great in the case of plywoodcore stock rotary cut veneer which is cut from the log nearer and nearerto its center where the wood becomes more knotty and the curvaturesharper. Difficulty with this source of waste has ben encountered in allprevious drying techniques known to me, including those involving use ofsupporting rolls. Cracking and warping are especially prevalent inconventional kiln or tunnel drying because in that case the veneer,being first cut into short lengths for stacking, must be stretched outflat for drying. A stack of these flattened horizontal pieces is formedon a kiln truck, each piece being separated from those above and belowit by a group of spacers or separator sticks.

In order to combat cracking and warping during drying it seemednecessary in the past, especially in kiln or tunnel drying, to carry outthe operation slowly at low temperature. Even then, however, muchwastage occurred, and at the low temperatures employed drying becameprolonged and more expensive.

In the light of these difficulties, for which, to my knowledge, therehas been no effective solution heretofore proposed, the primary objectof my invention is to provide a veneer drying process which, at minimumcost, practically eliminates objectionable Warping and cracking duringor as a result of drying of green veneer and at the same time enablesdrying at much higher temperatures and for a shorter time. 1

A further and related object is to accomplish the foregoing withoutrendering obsolete most existing or conventional drying apparatus inwhich the plywood industry has a large investment. Moreover, theapparatus or facilities required in addition to that already being usedcan be extremely simple and easily accommodated in existinginstallations. The initial cost and the continuing expense are bothnegligible by comparison with the savings effected.

In its broad aspect my invention resides in the discovery of a processuniquely advantageous for the rapid drying of green veneer, especiallyrotary cut soft wood veneer, such as Douglas fir, pine, spruce,cottonwood, etc. Since most hardwood veneers are sliced, instead ofbeing rotary cut by a peeler lathe, the problem of instant concern willnot ordinarily be sufficiently acute in that case to justify specialdrying practices.

My discovery essentially is that if a small quantity of oil is suppliedto rotary cut veneer as a plasticizer before or during drying it willnotbecome brash and crack or warp appreciably, but when dry will bepliable and flexible. It is discovered, moreover, that onlycomparatively minute quantities of oil need be carried to the veneer inorder to obtain these results. For instance, by entraining a quantity ofoil vapor in the heated air circulated through the dry kiln, or byapplying oil to the separator sticks, either by spraying them or dippingtheir ends,

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before utilizing them in stacking the veneer sheets on trucks to enterthe kiln, splitting and warping of the veneer during drying are greatlyreduced, if not altogether eliminated.

Oil may be supplied to the veneer in various alternative ways, the wordsupply as herein used being intended as a generic term to include allherein described techniques and equivalents thereof by which oilmolecules may be caused to come in contact with the veneer surface inaccordance with the invention. One technique involves dipping the veneersheet or strip end portions. The surface of the veneer may be sprayed orotherwise coated lightly, such as by brushing, either uniformly or inpatches or zones, as may be convenient. Veneer-permeating oil vapor maybe imparted to the kiln atmosphere by a process of evaporation of oilfrom troughs or other containers located preferably adjacent to theveneer stacks, or sprayed in a fine mist into the circulating air.Whether the stacks move slowly on trucks through the kiln, are shiftedperiodically, or remain stationary therein for drying in separatebatches, is immaterial. Where applicable, similar techniques may beemployed effectively if the veneer is carried continuously on rollsduring drying.

The exact scientific theory explaining the plasticizing effect of theoil on the veneer discovered is unknown to me, but the effectiveness ofthe process, as determined by repeated tests, is undoubted. It ispractically impossible to prescribe generally any minimum or maximumquantities of oil required, because some benefit is to be derived fromcomparatively minute, although appreciable, quantitics imparted to theveneer, while the benefit derived increases if a somewhat largerquantity of oil is imparted to the veneer. The veneer should not besaturated or completely coated with oil, however, or even infused withit to a substantial degree. On the contrary the oil application isessentially superficial in nature and the quantity of oil used should besufficiently small to avoid any perceptible difference in the surfaceappearance or color of the dried veneer over that dried in theconventional way.

Consider, for example, the technique in which oil is imparted to theveneer solely from oil-dipped separator sticks used between the veneersheets in stacking them on a kiln truck. These sticks are usually /2inch to 1 inch thick and, in supporting standard 26 /2 inch x 50 inchveneer sheets, are usually three in number, arranged paralleltransversely of the grain and length of the sheets, such sticks beingspaced approximately equally. The sticks are dipped momentarily in oil,surplus oil is allowed to drain from them, and then the sticks areinserted between veneer sheets. During drying a sufficient quantity ofoil is attracted by capillary action, or evaporation and absorption, tothe veneer from the separator sticks alone to plasticize it enough toallay virtually all tendency for the wood to crack and warp, thusdrastically reducing wastage.

Oil may be supplied or imparted more directly to the veneer foraccomplishing the same purpose, for example, by suspension of oil in theatmosphere, or by spraying or brushing oil directly on the veneer, asmentioned. The oil will then be distributed over the wood surface bycapillary action or by evaporation and absorption. The quantity of oilpreferred depends, of course, on the variety of wood and the type of oilused. Moreover, the quantity used by a particular manufacturer of veneerwill also depend on his concept of the desirable balance betweenreduction in cracking and warping of the veneer and the tolerableresidue of oil in the wood for the purpose for which the veneer is to beused. In the case of core stock veneer for plywood, a certain amount ofcracking is permissible whereas the surface layer stock must be freefrom cracks. Moreover, the quantity of oil required further depends in agiven case upon the drying temperature, because when temperature isincreased the wood tends to be rendered more brash and the plasticizingrole of oil application in preventing cracking is correspondingly moreimportant. In general, it may be said that any slight traces of oilfound naturally in the atmosphere of a dry kiln will have no noticeableeffect. However, an appreciable amount of oil deliberately suppliedgreases to the veneer by adding oil vapors to the atmosphere, or byapplying liquid oil directly to the veneer as indicated, will producethe striking effect I have observed and described herein.

Concerning oils useful in practicing the method, some are more effectivegenerally than others. In this regard, the properties of the particularwood, the drying ternperature and other factors influence the select1on.Dipping the separator sticks in any mineral 011 having an SAE viscosityrating of 40, for example, produces very satisfactory results in thedrying of Douglas fir veneer at 340 degrees F., in a tunnel type dryerwith such sticks interposed between the veneer sheets as spacers, but 1tis to be understood that lower or higher viscosity oils are also suitedto the method. Moreover, numerous types of vegetable oils can be used,such as linseed, castor, soybean, olive, cottonseed and other vegetableoils, the controlling consideration being their ability to produce onthe veneer the plasticizing effect described above. Moreover, oilsulphonated to render it water soluble may be utilized and may bepreferred for veneer to be bonded with glues not compatible with oil. Infact, because of the wide diversity in the types of oils capable of use,and permissible variations in their manner of use effectively in thepractice of my invention, it does not seem necessary or practicallyfeasible to attempt listing them all or stating any definite viscosityor quantity control limits applicable. The described use of separatorsticks as the oil carrier medium has proven to be an especiallysatisfactory technique.

Concerning drying temperatures, in the case of kiln drying, whereasDouglas fir veneer usually requires about 8 hours to be dried attemperatures of about 230 degrees R, my oil application veneer dryingprocess enables halving this drying time, for example, by raising thedrying temperature to the vicinity of 340 degrees F. without impairingthe product. In fact the oil treated veneer thus dried is less brash andless warped than unplasticized veneer dried to an equally low moisturecontent at the lower temperature and for the longer time mentionedabove.

In this specification the word veneer has been used throughout todesignate the material to which the method is particularly applicable,and the connotation of the term is that usually understood andinterpreted in the plywood industry. It designates thin wood sheetswhich in the manufacture of plywood vary ordinarily from one-tenth of aninch to one-fourth of an inch in actual thickness. Moreover, theinvention is restricted to the process of drying substantially greenveneer in which oil is made available for the wood at or near thebeginning of the drying process, such that as the wood becomes heatedand loses any appreciable amount of moisture, plasticizing occurs.

It will be understood that the mechanical procedure of drying, that is,whether the veneer is carried on rolls, or on trucks in a standard kilnor tunnel dryer, for instance, will be a factor influencing the choiceof oil and application technique. Kiln or tunnel drying naturally lendthemselves to a wider selection of methods for carrying oil to theveneer than that afforded by drying veneer in roll driers because in thelatter case the drying can be and is, therefore, usually more rapid thankiln drying, because each veneer layer is unconfined and the sheets arenot pressed flat during drying. Dissemination of oil to the wood duringroll drier veneer drying should be more rapid or immediate, therefore,to have maximum effect. Consequently, direct application of oil to theveneer, such as by brush or spray application, preferably at spacedlocations, is preferred in the case of drying veneer in roll driers, andmore oil and/ or less viscous oil may be desdirable.

I claim as my invention:

1. The process of drying veneer sheets which comprises initiatingheating of the veneer in an air atmosphere at a temperature above normalroom temperature while at least a major portion of the surface area ofthe veneer is substantially free of added oil, and, during continuedheating of the veneer, supplying for access to the surface of the veneeroil of a type normally liquid at room temperature, less volatile thanWater and substantially immiscible with water, and thereby deterringchecking and cracking of the veneer.

2. The process of drying veneer sheets which comprises applying to thesurface of the veneer at spaced locations over an area substantiallyless than half the surface area of the veneer oil of a type normallyliquid at room temperature, less volatile than Water and substantiallyimmiscible with water, and heating the veneer in an air atmosphere at atemperature above normal room temperature prior to dissemination of suchoil over a major portion of the surface area of the veneer.

3. The process of drying veneer which comprises heating the veneer in anair atmosphere at a temperature above normal room temperature andcarrying in suspension oil of a type normally liquid at roomtemperature, less volatile than water and substantially immiscible withwater, and thereby reducing materially the brashness of the veneerresulting from drying.

4. The process of drying substantially green, rotarycut veneer whichcomprises applying to a minor portion of the surface area of the veneeroil of a type normally liquid at room temperature, less volatile thanwater and substantially immiscible with water, and then heating theveneer in an air atmosphere at a temperature above normal roomtemperature during which heating the oil materially reduces thebrashness of the veneer.

5. The process of kiln drying substantially green, rotary-cut veneerwhich comprises stacking the veneer sheets with separator stripsinterposed between successive sheets, which separator strips carry oilof a type normally liquid at room temperature, less volatile than waterand substantially immiscible with water, and subjecting the stack ofveneer sheets to an air atmosphere at a temperature above normal roomtemperature while oil from the separator strips reduces the brashness ofthe veneer.

6. The process of drying veneer which comprises placing in physicalcontact with the veneer an element of porous material carrying oil of atype normally liquid at room temperature, less volatile than water andsubstantially immiscible with Water, and heating the veneer in an airatmosphere at a temperature above normal room temperature while oil fromsuch porous material element reduces the brashness of the veneer.

7. The veneer drying process which comprises placing rotary-cut veneersheets in a drying chamber, supplying to such chamber air heated to atemperature considerably above the boiling temperature of water, andsupplying to the wood of the veneer oil of a type normally liquid atroom temperature, less volatile than water, and sustantially immisciblewith water, said oil being supplied to the veneer in such relativelysmall amounts as to only spread over the surface layers of the veneerand not infuse into the veneer in any substantial degree, and therebycurtailing the brashness and preserving the flexibility of the veneer.

8. The process defined in claim 7, in which the oil is suspended in theair and carried into contact with the veneer by flow of the air over theveneer.

9. The process defined in claim 7, in which the oil is supplied to theveneer by application of the oil directly onto the surface of the veneerand over only a minor portion of the surface area of the veneer.

10. The process defined in claim 7, in which the oil is supplied to theveneer by placing in contact with such veneer a stick carrying the oilfrom which stick oil is transferred to the veneer.

11. The process defined in claim 7, in which the oil is an oil selectedfrom the group consisting of mineral oil, linseed oil, castor oil, soybean oil, olive oil and cottonseed oil.

12. The process defined in claim 7, in which the oil is mineral oilhaving an SAE viscosity rating of approximately 40.

13. The process of drying veneer sheets which comprises drying theveneer in an air atmosphere while at least the major portion of thesurface area of the veneer is substantially free of added oil, andduring such drying supplying oil of a type normally liquid at roomtemperature, less volatile than water and substantially immiscible withwater, for access to the surface of the veneer and thereby deterringchecking and cracking of the veneer.

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